The present invention relates to a pacing system, in particular an implantable pacemaker for pacing multiple chambers of the heart and sensing signals from the myocardium of multiple chambers of the heart which will allow the verification of a cardiac evoked response resulting from a delivered charge to the heart tissue comprising: a preamplifier system. The present invention further relates to a method implantable cardiac pacing particularly concerning multichamber pacing and sensing with the capability of performing capture detection in any chamber.
A cardiac pacemaker delivers an electrical stimulus through an implanted lead to the myocardium of a selected area of the heart. If the electrical stimulus is of sufficient energy, the area of myocardium will depolarise and cause a mechanical contraction. The depolarisation immediately following a delivered stimulus is referred to as an evoked response, and the ability to detect this signal is called capture detection or capture verification. This evoked response usually occurs 5-20 ms after the delivery of the stimulation pulse. Typically, the energy output of the pacemaker is set to a level much higher than needed to insure a mechanical contraction. This extra energy consumes battery life, and a system which monitors the energy level needed and provides optimal pacing energy is highly desirable. Monitoring of the needed energy level is critical because the energy threshold to cause a mechanical contraction may change over time. By monitoring each delivered pace for the presence of the evoked response, the System may respond to a change in the threshold level required for a contraction. Verifying an occurrence of an evoked response has been a technical challenge. Due to the characteristics of the tissue-electrode interface of an implanted electrode and the type of pacing waveform typically used, the evoked response signal is usually masked by a much larger polarization voltage which occur after a delivered pace. Many different techniques for overcoming this problem have been proposed. Most of the proposed solutions have not been used in actual practice due to implementation problems.
U.S. Pat. No. 4,858,610 details the use of charge dumping following delivery of the stimulating pulse to decrease lead polarization and also the use of separate pacing and sensing electrodes. U.S. Pat. No. 4,686,988 shows the use of a separate sensing electrode for detecting P-waves in the presence of an atrial stimulation pulse. U.S. Pat. No. 4,373,531 uses pre and post stimulation pulses to neutralize the lead polarization. U.S. Pat. No. 4,537,201 uses an anti-logarithmic amplifier to linearize the exponentially decaying polarization signal, while amplifying only the evoked response signal. U.S. Pat. No. 5,607,457 uses differential sensing between the electrodes in the atrium and ventricle. U.S. Pat. No. 597,957 teaches the sensed cardiac signal is added to either a differentiated or autocorrolated sensed cardiac signal and the difference between the two used to determine the presence of an evoked response. U.S. Pat. No. 5,843,136 teaches the use of an extra capacitor which is switched in series with the DC blocking capacitor to discharge the polarization charge quickly after a stimulus pulse. U.S. Pat. No. 5,443,485 describes a system using an input highpass filter, then bandpass filtering the signal, and finally integrating the bandpassed signal. Many of these approaches are difficult to implement or require pulses increasing battery current consumption. Signal integrity is also an issue with many of these solutions.
Therapies exist needing the capability of pacing multiple chambers of the heart. One pacing therapy for congestive heart failure relies on the placement of electrodes in the right atrium, right ventricle, and the left ventricle. The capability to analyse the signals from many different locations of the heart is also desired. As implantable pulse generators are being developed with greater and greater signal analysis capability, there is a desire to analyse the different signals from many electrode configurations.